Source Location of the Wedge-like Dispersed Ring Current in the Morning Sector During a Substorm M. Yamauchi, H. Nilsson, R. Lundin (IRF-Kiruna), P. C:son Brandt, Y. Ebihara, I. Dandouras, H. R

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Source Location of the Wedge-like Dispersed Ring
Current in the Morning Sector During a
SubstormM. Yamauchi, H. Nilsson, R. Lundin
(IRF-Kiruna), P. Cson Brandt, Y. Ebihara, I.
Dandouras, H. Rème, C. Vallat, P.-A. Lindquvist,
A. Balogh, and P.W. Daly
XY0863 (EGU06-A-05934) 2006-4-6 / see also XY0868
in this session.
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What is "wedge-like dispersion" / What is the
problem?
Sub-keV trapped ions seen almost all satellites
at around L4-6. They are wedge-like
energy-latitude dispersed as shown in both Viking
data (mid-altitude) and Cluster data (equatorial
plane). See Poster XY0868 by Yamauchi and Lundin
in this session. Data analyses and simulation
confirmed that they are drifting trapped ions
(I.e., Cluster should see the same phenomena as
Viking). Past analyses raised two main
questions. (1) Past identification of "wedge"
observed by Cluster was in-appropriate because
the resultant distribution does not agree with
the simulation or other satellites. We need to
refine the criterion. (2) Past statistics
suggests that the source can be in the morning
sector during substorms. We need to identify the
source location from event study.
westward drift eastward drift
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New criterion and statistics
Past criterion completely isolated from gt 5 keV
ring current. Þ Only (a) is identified as
"wedge" but not (b) or (c) New criterion
isolated from gt 5 keV component at the wedge
location, as long as wedge is extended from
sub-keV. Þ All of (a), (b), and (c) are
identified as "wedge" New statistics shows
morning peak, which is consistent with the other
statistics and simulation.
(b)(c)
(a)
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Case study
We confirmed the morning peak, suggesting that a
substantial amount of the wedge might be formed
in the morning. Pitch-angle distribution suggests
the ionospheric source (also consistent with
morning source). We have several possibilities
in terms of the location of dense ion formation
and location that dispersion starts
Ion source dispersion scenario
night night No !
night morning (A)
morning morning (B) (C)
(A) Strong electric field push ions quickly. (B)
Energetic ions precipitate and scatter
ionospheric ions into the space. (C) Unknown
local energization process. Þ Need to find
events when the wedge is formed during substorms.
We found one case. Wedge is seen only at
outbound.
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Relative spacecraft position
1 0 -1 -2
S/C-1
No
2001-10-21 2340-2400 UT
Yes
No
9.0 0.1 MLT
No
Yes
S/C-4
No
S/C-3
Yes
?
No
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Observation summary
time-of-flight principal

• S/C-1 (2345 UT), S/C-4 (2350 UT), and S/C-3
  (2340 UT) passed through the same magnetic flux
  tube at 9 MLT (L4).
• Wedge-like dispersion at 2350 UT.
• No low-energy signature at 2340 UT.
• Butterfly-trapped distribution
• Þ Bounce inside the geomagnetic bottle.
• Þ Difference between 2340 UT and 2350 UT in
  the same flux tube means
• an temporal variation although observation is
  made in the opposite hemisphere.
• VE eastward ExB drift speed energy
  independent, MLT dependent
• VB westward magnetic (ÑBcurvature) drift
  velocity energy dependent
• VE gtgt VB at low energy (lt100 eV) and VE VB at
  high energy (value depends on E-field strength).
  From dispersion curve, the last-coming ions are
  10-20 keV. Therefore,
• VE VB at 20 keV in the present case.

V1 VE-VB VE _at_ 0.1 keV V2 VE-VB ltlt VE _at_ 10
keV
0.1 keV 10 keV
V1 VE V2 VE-VB
t t?t

• V1t V2(t?t) or
• (t?t)/?t V1/(V1-V2)
• VE/VB (note VB_at_10 keV)
• (E/B)(qRB/3Wg)
• E mV/m/g or
• t ?tE mV/m/g - ?t
• for observation near equatorial plane, where
• E and B are the field strengths,
• q is the charge,
• R 4 RE is the geocentric distance,
• W 10 keV is the ion energy, and
• g 1, 0.9 0.7 for 90, 40 0 pitch angles

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Dispersion analysis
13 mV/m
Pitch angle of the "wedge" is about 4090
(g0.91.0) ? t (1.1EmV/s - 1) ?t
Electric field is 13 mV/m for half an hour ?
t 0.12.3?t VE 310 km/s (a) 0.1 keV
_at_ 2350 UT, S/C-1 Ü? Nothing _at_ 2340 UT, S/C-3
temporal chance (b) 10 keV _at_ 2353 UT, S/C-1
Ü? Nothing _at_ 2340 UT, S/C-3 temporal change
(c) 0.1 keV _at_ 2350 UT, S/C-1 Ü? 10 keV _at_ 2353
UT, S/C-1 temporal or spatial Combination of
(a) (b) it is temporal change ? ?t lt 13 min
? t lt 30 min before 2340 UT ? drift distance
VE t lt 20000 km ? dispersion started at 79
MLT. Combination of (b) (c) if temporal ?
?t 3 min ? t 0.58 min before 2350 UT ?
drift distance VE t 1005000 km ?
dispersion started at 89 MLT.
On the other hand, we observed O "wedge" at
0.05-0.3 keV (20 km/s 50 km/s). The 0.05 keV
O takes 2030 min to travel from the ionosphere
to the Cluster location along B in best case.
From this (1) Source timing is about 2030 min
before, i.e., at 23202330 UT. (2) The
combination (b)(c) cannot be true, i.e., the
observed dispersion is mostly the spatial
structure. (3) O should not have
mirror-bounced, and this is confirmed from nearly
uni-direction pitch angle.
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General context (deduced from ENA image)
ENA image indicates (1) strong E (2) No
energetic H in the late morning
sector (3) qualitative difference between
O and H Strong E ? scenario (A) in the Table
Ions lt 10 keV could have convected to the
morning sector quickly without forming the
dispersion. No energetic H ? difficult for
scenario (B) in the Table unless electron is
important H - O difference ? At least O wedge
can be formed local
post-midnight preference strong E (could be 10
mV/s)
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Summary and conclusions

• Cluster statistics (9 MLT) shows that
• (a) New identification criterion works (for
  Cluster).
• (b) Local time distribution suggests the morning
  source.
• (c) Pitch-angle distribution suggest ionospheric
  source (consistent with morning source).
• Case study from 2001-10-21 event (9 MLT) shows
  that
• (d) The "wedge" suddenly appeared in the magnetic
  flux tube in which no signature was recognized 10
  minutes before.
• (e) The dispersion is formed within 3 Re distance
  from the spacecraft within 30 minutes before the
  observation.
• (f) Observed oxygen ions of the "wedge" were not
  mirrored, i.e., they directly came from northern
  ionosphere 20-30 minutes before.